A block diagram of a receiver unit of a typical prior art wireless microphone system is illustrated in FIG. 2. In general, as shown in FIG. 2, the receiver has an antenna 1 for receiving signals from an FM transmitter remotely located from the receiver and not shown in FIG. 2. The received FM signals are supplied to a radio-frequency amplifier 2 that amplifies the low level FM signal. The amplified radio-frequency FM signal is then supplied to a mixer 4. The mixer 4 combines the radio-frequency signal with a fixed frequency from a local oscillator 3 to output an intermediate frequency. The intermediate frequency contains the audio information yet to be extracted. The intermediate frequency is amplified via an intermediate-frequency amplifier 5 and then supplied to an FM detector 6. The output of the FM detector is the originally encoded low-frequency audio signal.
The low frequency, audio signal, is originally encoded and transmitted in the logarithmically compressed state for the purpose of eliminating noise in the FM encoding and transmission process. After reception, mixing and decoding, the audio signal is then logarithmically expanded through a logarithmic expander 7 to restore its original dynamic range. Typically, the output of the logarithmic expander 7, (output terminal 8), is amplified by a power amplifier (not shown) and then supplied to a speaker unit (also not shown).
The receiver unit of the above type is typically built from various circuit components as shown in FIG. 3. There are four major parts of interest in the receiver unit:
1) Radio frequency amplifier 2,
2) local oscillator 3,
3) mixer 4,
4) intermediate frequency amplifier 5
As shown in FIG. 3, the (FM) radio-frequency amplifier 2 of the prior art generally comprises:
I) a field-effect transistor Q1 having 20 decibels (dB) amplifying gain at its operating point,
II) a double-tuned circuit 2a between the gate of the transistor and the antenna 1,
III) a double-tuned circuit 2b between the drain of the transistor and the mixer 4.
The local oscillator 3 comprises
I) a crystal resonator X1,
II) a transistor Q4 for resonating the crystal oscillator X1 at its third harmonic, and
III) a multiple-stage transistor Q5 for multiplying the third harmonic by another factor of three. Thus, the original frequency of the crystal resonator X1 is multiplied by a factor of nine and is then supplied to the mixer 4 as the local oscillator frequency.
The mixer 4 has a current-feedback bias type transistor Q2. Items 2, 3, and 4 typically constitute the front end of the receiver.
The intermediate-frequency (IF) amplifier 5, is provided with a current-feedback bias type transistor Q3. In FIG. 3, reference numerals 4a and 5a denote ceramic filters.
Applications for wireless microphone systems include use with musical instruments, such as electric guitars, and with video cameras. In the latter applications, the receiver unit is installed on or in the camera itself to receive signals from a remote transmitter. One significant drawback to the use of wireless microphones in electric guitars, video cameras and other applications is the relatively high cost of wireless microphones systems, as well as the large size and weight of the systems. Given these market constraints, the receiver unit of the system should be preferably compact, inexpensive and light. For example, with respect to use of the system with an electric guitar, it is necessary to reduce the manufacturing price of the system to a level much lower than that of the guitar itself. For those goals to be achieved, the receiver unit of the prior art system described above must be modified to reduce the number of components to be assembled without degrading its performance or electrical characteristics.